This invention relates to a portable gas dynamic spray gun for cold gas dynamic spraying of a metal, alloy, polymer or mechanical mixtures thereof.
Gas dynamic spray guns coat substrates by conveying powder particles in a carrier gas at high velocities and impacting the substrate to form the coating. The gas and particles are formed into a supersonic jet having a temperature below the fusing temperature of the powder material, and the jet is directed against an article to be coated.
One difficulty associated with some of the prior art spray systems is that the powder is injected into the heated main gas stream prior to passage through the nozzle. The powder has a tendency to plug a throat of nozzle to result in backpressure and attendant malfunction of the gun. This requires a complete shutdown of the system and cleaning of the nozzle. Larger particles tend to plug the nozzle even more.
The second difficulty is associated with low durability of the convergent and throat portions of nozzle. Because the heated main gas stream is under high-pressure, the injection of the powder also requires high-pressure powder delivery systems, which are quite expensive and would be difficult to use in a portable cold spray gun.
Some known spray guns use a powder feeding system having an enclosed hopper for containing powder in loose particulate form. A carrier gas conduit connected to a carrier gas supply extends through the hopper in its lower portion and continues to a point of powder-carrier gas utilization. Fluidizing gas in a regulated amount is supplied to the hopper and the flow of the fluidizing gas is regulated by sensing the pressure at a point in a carrier gas line, which pressure is responsive to the mass flow rate of solids, and then using the change in the pressure in the conveying gas line, if any, to regulate the flow of the fluidizing gas. This type of system has certain problems with control and uniformity of the powder feed rate. One such problem is pulsation, apparently due to a pressure oscillation, resulting in uneven coating layers.
Another problem with some of the known spray guns relates to the heating unit for heating the carrier gas prior to the nozzle. Generally, the heating unit is either too large to be used in a portable spray gun, or it is too small to heat the carrier gas sufficiently.